In modern machine tools for milling and drilling work, in which a plurality of machining operations are performed automatically according to a programme stored in the electronic controller, using different tools, a series of monitoring and checking functions are of course necessary in order to recognise possible sources of error in good time and to indicate and if possible also eliminate them. An important role is played in practice by the detection of tool breakages, and in particular of drill breakages. Thus if a broken drill tip remains in the drilled hole and this hole is further machined, e.g. widened out with a further tool in a subsequent machining operation, this subsequent tool encounters the tool tip left in the hole with the result that this subsequent machining operation cannot be performed and also that the further tool may break. Furthermore, since several machining operations are often performed with one and the same tool on the same or succeeding work-pieces in an automatic machining sequence, it is necessary to ensure that each of these tools remains in an operative condition.
Because of the facts set forth above various systems for checking for breakage of drills and other tools have been developed, in particular for universal drilling and milling machines, by which each of the various tools can be measured before and after each machining operation. The various measurements are electronically compared either in the numerical control device of the machine tool or in a special device, and, if there are substantial deviations, marking of the tool concerned takes place in the control, possibly coupled with an optical or acoustical sign, by which, for example, the further use of that tool is prevented.
As a rule electro-optical systems are used for monitoring tool breakages, with their light sources and photo-cells arranged in the immediate working area of the machine tool so that the particular tool is illuminated over a predetermined length and the light intensity detected by the photo-cell is evaluated as an index of the condition of the tool concerned. In practice however serious disadvantages have been found with systems operating on electro-optical principles. Thus since the tools used can have very varied lengths and diameters, correspondingly large spatial ranges of measurement must be monitored with the same degree of accuracy. Furthermore the turnings and drilling fluid often lead to troubles in the checking system, since the turnings and/or liquid droplets that may for example cling to the cutting edges of the drill can lead to false readings of the measurements. Particular difficulties occur in the case of very long, thin drilling tools.